MARRIOTTSVILLE, Md., March 12 (Reuters) - The gunman who killed two store employees at a Maryland mall early this year may have been obsessed with the 1999 Columbine High School massacre and left a journal behind in which he wrote that he might already be dead, police said on Wednesday.

The shooter, Darion Marcus Aguilar, 19, did not know the victims when he opened fire at a crowded Columbia, Maryland, mall about 20 miles (32 km) west of Baltimore on Jan. 25 before killing himself with his 12-gauge Mossberg shotgun.

Investigators have been unable to find any connection between Aguilar and the employees he gunned down at a Zumiez Inc skateboard store, the Howard County Police Department's investigation found.

"Nobody saw this coming. He was a young man that did not garner much attention to himself," Police Chief William McMahon told a news conference.

A search of his computer suggested that Aguilar may have had a fixation with the massacre at Columbine High School in Colorado, where two teenagers shot a teacher and 12 classmates to death before committing suicide, police said.

Aguilar, who worked at a doughnut shop, waited inside the mall for more than 40 minutes before starting the attack. Police said he may have wanted to begin shooting at the same time that the Columbine massacre started, 11:14 a.m.

"Aguilar's weapon, attire, backpack containing explosives, and method of suicide were all similar to those used in the Columbine incident," a police statement on the investigation said.

After assembling the shotgun inside the store's dressing room, Aguilar took a picture of himself in the mirror holding the gun.

He posted it on Tumblr, a social networking website, along with a text that said, in part, "Today is the day."

Police declined to release the photo.

Investigators found thousands of searches on his computer for mass murder, homicidal thoughts, school shootings, gun shops, shotguns and mental illness.

A journal found at Aguilar's home in College Park, Maryland, had references to death, suicide and killing people, police said.

One of the entries said, "I think I may already be dead."

Detectives found that he had seen a doctor in 2013 and had been referred to a psychiatrist, but there was no record he scheduled or went to an appointment, police said.

The investigation pointed to the need for a wider discussion about the stigma of mental illness, McMahon said.

The attack was the latest in a spate of U.S. shootings. It prompted renewed questions about the vulnerability of public places and fresh calls from some politicians for stiffer gun control. (Writing by Ian Simpson; Editing by Scott Malone and Jonathan Oatis)

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